Oil heater



FIG. 1 is a front, top and right side perspective view of an oil heater showing our new design;

FIG. 2 is a rear, bottom and left side perspective view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a left side elevational view thereof; and

FIG. 5 is a right side elevational view thereof. 

The ornamental design for an oil heater, as shown. 